Fig. 2

Single-molecule sensitivity. a Representative SERS spectra from a real-time BiASERS of a mixture solution of 1 × 10⁻⁷ M ¹⁴N-adenine and ¹⁵N-adenine in 10 mM KNO₃. Spectrum 1 is averaged from all spectra, spectrum 2 refers to a mixed event of both adenines, spectrum 3 refers to a single-molecule event of ¹⁴N-adenine, and spectrum 4 refers to a single-molecule event of ¹⁵N-adenine. All spectra were taken at +0.5 V and 0.1 s. The two reference spectra 5 and 6, one was taken from a different solution with only ¹⁴N-adenine (10⁻⁷ M in 10 mM KNO₃), acquired for 0.1 s and the other was taken from the same mixture solution but with a prolonged acquisition of 0.5 s. b The effect of acquisition time on the distribution of the peak wavenumber. Prolonging the acquisition time from 0.1 to 0.5 s, the dual-peak distribution of the peak wavenumber becomes a single-peak distribution (2500 spectra for each). c Concentration effect on the distribution of the FWHM. FWHM of Raman bands from samples (1000 spectra taken at 0.1 s) at a lower concentration of 10⁻⁷ M is smaller than that from a higher concentration sample of 10⁻³ M. Both the spectral merging and broadening of bands indicate the transition of single-molecule to many-molecule sensing. d, e The contour maps of SERS of a typical asynchronous blinking of the mixed isotopic adenines (d) and a typical fluctuation of the single adenine (e) recorded in 5 s, respectively in single-molecule sensing. The excitation (785 nm, 8 mW), the applied voltage (+0.5 V) and the nanoslit in use were the same for all experiments